1. Field of the Invention
This invention relates to carrier transmission systems and, more particularly, to automatic signal level control circuits for such systems.
2. Description of the Prior Art
Telephone carrier systems such as those described in L. Q. Kraisen et al U.S. Pat. No. 3,510,584, granted May 5, 1970, P. J. May et al U.S. Pat. No. 3,601,538, granted Aug. 24, 1971, and J. Garcia et al U.S. Pat. No. 3,700,817, granted Oct. 24, 1972, include carrier modulation and demodulation circuits for modulating and recovering voice signals in single channel carrier systems. Due to the wide variation in signal levels in such systems, it is necessary to provide automatic gain control circuits to control the level of received signals. It is customary in such automatic gain control circuits to recover the direct current component of the received signal and use that direct current component to control the level of the signal received.
If the audio frequency components of the detected signal are not completely removed from the AGC signal, second harmonic distortion occurs, limiting the quality of the received signal. This second harmonic distortion can be minimized by providing a very large filtering capacitor in the AGC circuit. Unfortunately, however, such a large capacitor lengthens the response time of the AGC circuit and makes it difficult for the receiver to track transient supervisory signals such as dial pulsing, ringing, and off-hook signals.